Structure and Anatomy of the Heart

The heart is central to the working and functioning of the human body, as it is responsible for maintaining the flow of the vital constituent, oxygen, to all parts of the body. By virtue of the crucial nature of its role, the heart has a very complex and intricate anatomy, ideally suited to its integral role and incessant activity.

The human heart, which is positioned in the middle mediastinum, is in fact the muscle that works the hardest in the human body. In addition, it is an involuntary muscle, that is, it does not cease to work even when the body is at rest. In addition, its activity is not under the voluntary control of an individual. 

This muscular organ is responsible for the collection of impure or deoxygenated blood from all parts of the body, which after purification in the lungs and conversion to oxygenated blood, is ‘pumped’ to all parts of the body. All parts of the heart work together to maintain the cardiovascular system.

The adult heart beats at a rate of about 60 – 80 beats per minute, bringing it to an average of about 100,000 beats per day! On a daily basis, the human heart pumps about 7.2 liters of blood through the whole body.

Anatomy of the Heart

The heart, which is almost conical in shape, is encased in a covering called the pericardium. The dimensions of the heart are approximately 12 x 8.5 x 6 cm, and its weight is about 310g in males and approximately 255g in females.

Orientation and Surfaces

The apex of the heart is aligned in an anterior-inferior direction, which contributes to the heart having 5 surfaces.

• Anterior (or sternocostal): Right ventricle
• Posterior (or base): Left atrium
• Inferior (or diaphragmatic): Left and right ventricles
• Right pulmonary: Right atrium
• Left pulmonary: Left ventricle

Separating the surfaces of the heart are its borders, which include the following 4 major borders

• Right border: Right atrium
• Inferior border: Left ventricle and right ventricle
• Left border: Left ventricle (and a portion of the left atrium)
• Superior border: Right and left atrium, the great vessels

Layers of the Heart Wall

The wall of the heart comprises three well-defined layers contained in the pericardium. They are the endocardium, myocardium, and epicardium.

Epicardium: It forms the outermost layer of the heart wall.

Myocardium: This is the muscular middle layer of the heart wall, made up of involuntary striated muscle, and includes excitable tissue and the conducting system.

Endocardium: This is the innermost layer of the wall of the heart. It lines the valves and cavities in the heart. It is structurally similar to the endothelium that lines the blood vessels.

• A subepicardial layer: This layer joins the myocardium and epicardium
• A subendocardial layer: This joins the myocardial and endocardial layers. The Purkinje fibers are found in this layer

The Chambers of the Heart

Septa divides the heart into two halves, right and left. Each of these halves is further subdivided by a constriction into an upper cavity, known as the atrium and a lower cavity called the ventricle. Hence, there are 4 chambers in the heart: two atria and two ventricles.

Deoxygenated blood from the body reaches the chambers and oxygenated blood is distributed back to the body. The atria, which have thin walls, receive the blood coming to the heart through veins, and the ventricles, which have thick walls, pump blood away from the heart. The variation in thickness is due to the amount of myocardium present in the walls of the heart, which is directly related to the force required for the contraction of the chambers.

The blood which reaches the heart first enters the atria, and from here it moves into the ventricles. Blood in the left ventricle then passes into the aorta, from where it gets into the systemic circulation. In the right ventricle, blood is introduced into the pulmonary circulation by means of pulmonary arteries.

The right and left atria are separated by the interatrial septum and the ventricles are separated by the interventricular septum.

Right Atrium

• It receives incoming blood utilizing the superior and inferior vena cavae and the coronary veins
• From here, the blood is pumped into the right ventricle, passing through the tricuspid valve
• The inner surface of the atrium is split into two regions, the sinus venarum and the atrium proper, by means of a ridge known as the crista terminalis
• In the right atrium, there is a small oval-shaped concavity, known as the fossa ovalis

Left Atrium

• Oxygen-rich blood is received in the left atrium through the four pulmonary veins
• From here, the blood is pumped into the left ventricle, via the mitral valve
• The left atrium is divided into the inflow portion and the outflow portion

Right Ventricle

• Deoxygenated blood from the right atrium is pumped into this chamber
• From here, the blood enters the pulmonary artery, whose orifice is guarded by the pulmonary valves
• The right ventricle is triangular shaped and is separated by the supraventricular ridge into an inflow and outflow section
• An array of irregular elevations, the trabeculae carnage, is seen along the inner side of the inflow portion
• The outflow portion, on the other hand, is smooth-walled

Left Ventricle

• This chamber receives oxygen-rich blood from the right atrium
• This oxygenated blood then moves into the aorta, by means of the aortic orifice, which is guarded by the aortic valve
• Similar to the right ventricle, this chamber has an inflow and an outflow portion
• The inflow portion, as in the right ventricle, is lined by trabeculae carnage, giving it a sponge-like appearance

The Valves of the Heart

The main function of the valves in the heart is to direct the forward flow of blood and prevent its backward flow. Heart valves guard the outflow from each chamber and ensure that the blood flow is in one direction only.

The heart has four valves, categorized into two:

• Atrioventricular Valves – Located between the atria and the corresponding ventricle, they comprise the tricuspid valve and the mitral (bicuspid) valve
• Semilunar Valves – Positioned between the ventricles and its corresponding artery, they include the pulmonary valve and the aortic valve

The mitral and tricuspid valves are attached to the valve cusps by chordate tendineae, which are, in turn, connected to papillary muscles which hinder the prolapse of the leaflets of the valve into the atria.

The valves and chambers of the heart and the blood vessels connected to it can be well represented in a diagram of the human heart.

Blood Supply

Like all the organs in the body, the heart, too, requires blood which provides nutrients and oxygen. This is known as coronary circulation and comprises the veins and arteries that carry blood to and from the heart. The name ‘coronary’ is owing to the way in which the coronary arteries encompass the heart, very similar to a crown.

Two coronary arteries supply the heart:

• Left main coronary artery: It divides into two branches and provides blood to the anterior two-thirds of the interventricular septum and the adjacent part of the left ventricular anterior wall and the lateral and posterior portions of the left ventricle
• Right main coronary artery: Its branches serve the right ventricle, right atrium, and left ventricle’s inferior wall

The venous drainage of the heart is mediated by the coronary sinus, seen on the posterior aspect of the heart. Blood from the cardiac veins is channeled into the coronary sinus, from where it drains into the right atrium

The main branches of the coronary sinus are:

• Great cardiac vein
• Small cardiac vein
• Middle cardiac vein
• Posterior cardiac vein

Aorta

The largest artery in the human body is the aorta, which is responsible for the transport of oxygen-rich blood from the left ventricle to all parts of the body. Blood flow in the aorta is regulated by the aortic valve, which is made up of three cusps, thus ensuring that there is a unidirectional flow of blood.

The aorta, which is approximately a foot long, is divided into four parts. These four parts can easily be identified on a diagram of the human heart:

• The ascending aorta: Rising up from the heart, it is about two inches in length. The coronary arteries, which provide blood to the heart, are branches of the ascending aorta
• The aortic arch: This angles over the heart, bifurcating to give rise to blood vessels that supply the head, neck, and arms
• The descending thoracic aorta: Traverses down the chest, its small branches supplying the ribs and a few chest structures
• The abdominal aorta: Commencing at the diaphragm, this part of the aorta then divides into two iliac arteries in the lower abdomen

Nerve Supply

The medulla oblongata mediates the primary control of the heart. The cardio regulatory center of the medulla oblongata regulates cardiac activity. The sympathetic and parasympathetic parts of the autonomic nervous system make up the nervous supply to the heart. The pressor center also called the cardioacceleratory center gives rise to the sympathetic fibers, while the depressor center, otherwise called the cardioinhibitory center, gives rise to the parasympathetic fibers. The pressor and depressor centers are together termed the cardio regulatory center which regulates control over all parts of the heart.

• The sympathetic nervous system activates the sinoatrial node, thus, increasing the heart rate
• On the other hand, the parasympathetic system operates conversely to reduce the heart rate
• The sino-atrial node is the natural pacemaker of the heart and it does not require a nervous supply to execute its function

The Conducting System of the Heart

The unique anatomy of the heart encompasses the cardiac conduction system that activates and integrates contractions of the myocardium. This is made up of:

• Sinoatrial node
• Atrioventricular node
• Atrioventricular bundle (bundle of His)
• Purkinje fibers

Components of the Cardiac Conduction System

The cardiac conduction system is the most important part of the anatomy of the heart.

• Sinoatrial Node: Found in the upper wall of the right atrium, these cells can initiate electrical impulses spontaneously. The SA node is regulated by the autonomic nervous system
• Atrioventricular node: Positioned inside the atrioventricular septum
• Atrioventricular bundle: Also called the Bundle of His. It is divided into the right bundle branch and the left bundle branch
• Purkinje fibers: Seen in the subendocardial surface of the walls of the ventricles

Diseases of the Heart

• Cardiac failure: Congestive cardiac failure, left ventricular failure
• Disorders of heart valves: Stenosis and regurgitation
• Ischemic heart disease: Angina pectoris and myocardial infarction
• Rheumatic heart disease: Acute and chronic rheumatic heart disease
• Infective endocarditis: Heart infection due to bacteria, fungi, etc
• Cardiac arrhythmias: Sinus bradycardia, sinus tachycardia, asystole, fibrillation, heart block
• Congenital abnormalities: Atrial septal defect, Fallot’s tetralogy, patent ductus arteriosus, coarctation of the aorta

Conclusion

The anatomy of the heart, though complex, is the epitome of the perfect pump. It is designed to last one’s entire lifetime, yet, cardiac disease and coronary defects are quite common today. Improper eating habits, lifestyle, and other systemic illnesses very often lead to hardening of the coronary vessels and heart valves, rendering them unable to perform their functions at an optimum level. A precise understanding of the various parts of the heart is essential to understanding the physiology of the heart.